A major concern in any communication network is the control of transmission errors. Redundancy is the basis of error control. Automatic Repeat Request (ARQ) schemes are often preferred to Forward Error Correction (FEC) schemes because they are easy to implement and show superior performance at low bit-error rates. The throughput rate of an ARQ scheme strongly depends on the number of requested retransmissions and this falls rapidly with increasing bit error rate (BER). For applications involving real-time video and voice over very high speed networks, ARQ-based schemes impose an unacceptably long transmission delay. The problem is more acute if the data paths are fairly long, as in wide area networks (WANs). On the other hand, since FEC has no retransmissions, the throughput rate of FEC schemes is more or less constant, set by the code rate and to a large extent, independent of the channel error probability. Recovering lost data reduces the need for data retransmissions and enhances the quality of real-time application (video and voice) that cannot rely on acknowledgments (ACKs) and retransmissions because of the large delays involved.
High speed networks, such as Asynchronous Transfer Mode (ATM) and other WANs, are based on the fiber-optic links with very low bit error rates, typically from 10.sup.-10 to 10.sup.-14, over long distance fiber cable. The main cause of data loss in these networks is buffer overflow during congestion, rather than bit errors. In ATM networks, low priority cells are dropped first in the event of congestion. Since a lost cell is just a block of bits in error, essentially some form of FEC is needed.
At present, apart from the techniques of error concealment, there is only the recommendation of the International Telecommunications Union (ITU-T) for error recovery in ATM transmission. ITU-T recommendation I.363 `B-ISDN ATM Adaptation Layer Specification` Rev 1 Geneva 1993. This technique uses either a (128,124) Reed-Solomon (R-S) Forward Error Correction (FEC) method with interleaving or a (94,88) R-S code with interleaving. The first method recovers 4 lost cells in a group of 128 with a 3.1% overhead, while the second can recover 1 lost cell in a group of 16 with a 6.8% overhead. These techniques are specifically for end-end recovery.